A horizontal thrust fault situated at the Earth's surface does not excite any seismic radiation. Because of this and because it provides a satisfactory fit to the data, Kanamori and his co-workers have used a point force rather than a conventional moment tensor to represent the long-period Love- and Rayleigh-wave radiation from a number of shallow landslide sources. The force is supposed by Newton's third law to be ω2MD, where ω is the angular frequency, M is the slide mass, and D is the displacement. Day and McLaughlin (1991) have recently shown that the spall accompanying an underground explosion can be represented either by a shallow horizontal tension crack or by a vertical surface point force ω2MD, where M is the spall mass and D is the crack separation. Using their method, we show that a landslide can be represented in the JWKB approximation either by a shallow double couple or by a horizontal surface point force; for a Love wave the force is FL = ω2MD(1 − β20/c20), whereas for a Rayleigh wave it is FR = ω2MD(1 − 8β20/3c20), where β0 is the shear-wave velocity within the slide mass and c0 is the phase velocity of the surface wave in the vicinity of the source. The sliding block appears to be mechanically decoupled from the rest of the Earth, so that FLFR ≈ ω2MD, because of the reduced shear velocity β0 within the brecciated rockmass.

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